Toast the walnuts: Heat oven to 350° and place roughly chopped walnuts on a parchment-lined sheet tray. Place in oven for roughly 5 minutes (keep an eye on them, oven times vary). Remove and let cool.
Caramelizing the banana: Peel and slice the banana. Heat a sauté pan to medium/medium high with a tablespoon of butter. Wait for the butter to melt, then place banana slices in the pan so they lay flat. Without moving them, sauté for about 4-5 minutes. You can peek under them with a fork to see if they're browning enough. Flip them over and brown on the other side, another 3-4 minutes. Once done, turn heat off and scoop bananas on a plate or bowl and mash with fork. Set aside.
In a large mixing bowl, cream together the sugars (dark brown and cane) with the butter with a handheld mixer. Once fluffy and paler (~3 minutes), add the caramelized banana, heavy cream, vanilla extract and egg. Mix together.
In a separate bowl, mix the dry ingredients together (flour, cocoa powder, baking powder, salt).
In two batches, fold in the dry ingredients to the wet ingredients and mix together until just combined.
Switching to a spatula, mix in the toasted walnuts and chocolate chips until just combined. Cover the bowl and store dough overnight (ideally).
Once dough is refrigerated, heat oven to 350° and line two baking sheet trays with parchment paper (or use 1 at a time, storing the dough in the fridge in between bakes).
Take a tablespoon cookie scooper and scoop the dough onto a parchment-lined tray. You should fit 12 cookies on one tray, leaving 3-4 inches of space in between cookies.
Bake for 12-13 minutes. To achieve that classic flat cookie look, take the tray out of the oven after 10 minutes and tap it a few times on the stove (careful not to tilt the tray!). Place back in oven for remaining 2-3 minutes.
Once cookies are baked, remove and let cool on a cooling rack. Enjoy! These are best when eaten the day of but can also be store for 2-3 days in an airtight container.